This invention relates to an apparatus and a method for determining the mass concentration of cellulose fibers in a water suspension and more particularly to an on-line apparatus and a method for measuring and monitoring the consistency of a flowing pulp suspension.
The pulp and paper industry has several requirements to measure and to monitor the concentration of undesolved fibrous cellulose materials in suspension in a pulp-water mixture to determine what is generally known as the consistency of the pulp suspension. For example, the consistency of a pulp suspension is an important factor for the characteristics of the paper made from the suspension and has to be within rather narrow limits prior to applying it to the paper making machine.
The consistency of the pulp suspension is also an important factor in connection with measuring and monitoring the quality of the refining process as disclosed in our copending application Ser. No. 852,817 filed on Nov. 18, 1977 for "Apparatus and Method for Measuring the Degree of Refining of Pulp Fibers in the Preparation of Furnish for Paper Making". For use in measuring the quality of the refining process, it is necessary to utilize a known consistency which is much less than the consistency of the pulp suspension being refined, and which is usually of the order of 0.1% by weight. To assure reliable quality measurements, it is necessary that the consistency is measured and continuously monitored.
Since consistency is an important characteristic of the stock, many attempts have been made in the past to develop an on-line or flow technique to perform this measurement, but each of these has met with limited success. One prior art technique used to determine consistency is to measure the viscosity of the pulp suspension, but this approach is limited by uncontrolled variables, such as particle size and type, degree of pulp refining, degree of flocculation, surface conditions of the particles and concentration of inorganic additives.
Another prior art technique used to determine consistency has been the utilization of the optical transmission and reflection characteristics of the pulp suspension. However, this approach again was influenced by particle color, size and size distribution, the refractive index of particles, and by dissolved color. This technique is disclosed in U.S. Pat. No. 3,498,719 which issued on Mar. 3, 1970 to Wing et al for "Photoelectric Consistency Indicator for Pulp".
Another approach taken by the prior art to determine consistency is the utilization of the optical activity of the pulp suspension, and this approach has generally been much more successful than the other approaches. U.S. Pat. No. 3,518,003, which issued on June 30, 1970 to Meyn for "Procedure for Continuous Registration of the Concentration of Fiber Solutions" discloses the transmission of a plane polarized light beam through the pulp suspension and through a polarization analyzer having its transmission axis oriented in a selected direction with respect to the plane of polarization of the transmitted beam for reception by a photoelectric detector. The output signal from this photoelectric detector, and the signal from another photoelectric detector which receives the plane polarized light beam transmitted through the pulp suspension without transmission through the analyser, are processed to provide a signal which gives an indication of the pulp suspension consistency. However, this method, which relies only on one signal taking into account the optical activity of the pulp suspension and which largely ignores the effect of scattering in the suspension, has been found wanting in accuracy and repeatability and has been unsatisfactory from a calibration point of view.
It is therefore an object of the present invention to provide an apparatus and a method for determining the consistency of a pulp suspension which can easily and readily be calibrated to become independent of the transmission coefficient of the analyser and the characteristic of the detector providing the output signals.
It is a further object of the present invention to provide an apparatus and a method for determining the consistency of a pulp suspension which takes into account the imperfection of the polarizer and the analyzer, such as leakage, to provide accurate and repeatable measurements.
It is still a further object of the present invention to provide an apparatus and a method for determining the consistency of a pulp suspension which takes into account the reflection of light in a scattering medium.